1. Field of the Invention
The present invention relates to apparatus for image formation, and in particular to image forming apparatus for projecting real holographic images in space.
2. Description of the Prior Art
Systems capable of forming images from an illuminated object where one or a plurality of optically superimposed objects are projected into space find numerous and important applications in areas such as advertising and marketing, product exhibition, aircraft flight training and spacecraft simulation, as well as a host or other imaging applications.
One such system described in U.S. Pat. No. 3,940,203 issued Feb. 24, 1976, to LaRussa, which employs a reflection-type holographic analog of a spherical mirror and a birefringent array of optical elements. In this patent, the primary image source is linearly polarized and made incident upon the holographic mirror analog. The light from the primary image passing through the analog is provided a circular polarization by a first quarter wave plate. A fraction of this light is reflected by a beamsplitter where it is collimated and transmitted through the first quarter wave plate, beamsplitter, second quarter wave plate, and second polarizer. The result is then viewed by an observer.
This and other prior art relies upon the inclusion of two beam splitting surfaces within the system. Since the prior art's beam splitting surfaces are not preferential to any polarization state, a large portion of that light is indiscriminately lost within the system. Thus, the prior art requires either an extremely bright source image or, conversely, very dim ambient viewing conditions.
Other prior art relies upon an obliquely placed beamsplitter and concave reflector to produce floating images. Significant efficiency problems also exist as the source light is made incident upon beamsplitting surfaces. Additionally, the field of view characteristic of this art is constrained by the necessity to project the image beyond the obliquely place beamsplitter. Prior art of this configuration is also inherently large due to the basic geometry of the elements. Additionally, that prior art allows an observer to "look into" the device from certain vantage points and discern a "direct view" of the source of the real image.